When you run smbd -V on your Snow Leopard installation, you'll see it's running SAMBA version 3.0.28a-apple. While I'm not sure how much difference the "-apple" makes, version 3.0.28a is old. Very old. In other words, it's riddled with bugs. Apple hasn't updated SAMBA in 3 years, and for Lion, they're dumping it altogether for something homegrown. The reason? SAMBA is now GPLv3.